The present invention relates to sensors for direct current (DC) motors, and, more particularly, to a sensor circuit having a comparator responsive to back Electromotive Force (EMF) signals from a multi-phase brushless DC motor.
One method known in the art to control a three-phase brushless motor is to detect the zero-crossing event between a floating or released phase of the motor and the neutral phase. When the voltage on the released phase is equal to the neutral phase, the next phase is commutated after a predetermined delay period. Each phase is in turn driven, tied to ground and released. The output of a comparator on each of the phases is used for comparison to the neutral voltage. One problem with this method is the voltage offset from one comparator to the next may be significant, especially at low voltages and in an electrically noisy environment resulting in inconsistent or ambiguous comparisons.
Other methods use relatively complex or expensive circuits to determine the phase angle of the rotor using rotor angle sensors or phase angle detectors. Additionally, from an initial position, the DC motor may not start when power is applied if the motor is aligned with the actuated phase, the motor may start in the wrong direction, or the motor may rotate too slowly resulting in insufficient back EMF for comparisons.
Accordingly, it is desirable to provide a sensor for a brushless DC motor which is reliable, simple and relatively inexpensive to produce, particularly adapted for use in radio controlled model vehicles, such as airplanes, helicopters, boats and cars, and which senses the back EMF of the DC motor.